Field of the Invention
The present invention relates to pneumatic tires. Specifically, the present invention relates to run flat tires including load support layers.
Description of the Related Art
In recent years, run flat tires including load support layers inside sidewalls have been developed and widespread. Highly hard crosslinked rubber is used for the support layers. This run flat tire is called a side reinforcing type. In the side reinforcing type run flat tire, if the internal pressure is reduced due to puncture, a load is supported by the support layers. The support layers suppress flexure of the tire in a punctured state. This run flat tire allows for running for some distance even in the punctured state. Running in the punctured state is referred to as run-flat running.
During run-flat running, each bead portion repeats deformation and restoration. This causes heat generation in the bead portion. When run-flat running is performed for a long period of time, damage of the bead portion may occur due to the heat generation and the deformation. Run flat tires are desired to have high run-flat durability such that damage of each bead portion does not occur even during run-flat running for a long period of time.
The load support layers may cause an increase in the weight and the rolling resistance of the tire. In addition, highly hard load support layers increase the vertical stiffness constant of the tire. This may deteriorate ride comfort during normal running. If the sizes of the load support layers are reduced, these problems can be improved. However, the small-sized load support layers may impair run-flat durability.
There is a method of forming the load support layers from a rubber having a low heat build-up property, thereby suppressing heat generation to improve run-flat durability. In addition, there is a method of enhancing the heat release performance of a tire, thereby suppressing temperature rise of the tire to improve run-flat durability. Examination regarding a run flat tire whose temperature rise is suppressed is disclosed in JP2013-60162 (US2014/0305567).
There is a method of increasing the number of carcass plies or adding a reinforcing layer to each bead portion, thereby improving run-flat durability. Examination regarding a tire including a reinforcing layer provided in each bead portion is disclosed in JP2011-506199 (US2011/0000597).
Further improvement of run-flat durability with favorable ride comfort during normal running and low rolling resistance being maintained is desired. When the load support layers are formed from a rubber having a low heat build-up property, the strength of the load support layers may be decreased. An increase in the number of carcass plies and insertion of a reinforcing layer to each bead portion may cause an increase in the weight and the vertical stiffness constant of the tire. This may deteriorate ride comfort and rolling resistance. The method of enhancing the heat release performance of the tire is effective for improvement of run-flat durability. In addition to this, by improving the structure of the tire, further improvement of run-flat durability can be achieved.
An object of the present invention is to provide a pneumatic tire having improved run-flat durability, while favorable ride comfort during normal running and low rolling resistance are maintained, by improving the structure of each bead portion.